1. Field of the Invention
The present invention relates to analyzing liquid samples and more specifically to controlling the absorption of a liquid sample by an absorbent layer, thereby reducing the effect of hematocrit.
2. Background Information
Test strips are widely available for conveniently analyzing liquid samples. Typically, a test strip has one or more layers of absorbent material containing chemical reagents. When the liquid sample is applied to the absorbent layers, the reagents react with the components of interest in the test sample. The resulting reaction can then be measured by a variety of means to yield a measurement of the components in the sample.
For example, it is useful for certain diabetes patients to monitor the concentration of glucose in their blood. A glucose test strip can have an absorbent layer containing reagents that react with glucose present in the blood sample. When a patient applies a blood sample to the absorbent layer, the reagents then react with the glucose in the sample. If the reaction results in a detectable color changexe2x80x94turning the absorbent layer from colorless to dark blue, for examplexe2x80x94the reaction can then be measured, and the amount of color produced related to the concentration of glucose in the blood sample.
A patient will typically use the test strip with a separate device that can measure the chemical reaction on the absorbent layer and display the patient""s blood glucose concentration. One such device uses a reflectance meter, which measures reflected light of specific wavelengths, to monitor the light reflected from the surface of the absorbent layer. As the chemical reaction in the absorbent layer causes the color change, the reflectance meter can monitor the reaction by detecting changes in the light reflected from the surface of the absorbent layer. For example, as the chemical reaction proceeds, the reflectance meter can measure an increase of blue light reflected from the absorbent layer surface. Thus, the reflectance of light from the absorbent layer can be used to monitor the chemical reaction and thereby determine the glucose concentration in the blood sample.
However, these reflectance measurements can be subject to variability, leading to inconsistent results. One factor is the volume of liquid sample applied to the absorbent layer. Ideally, when the sample is applied to the absorbent layer, it should be thoroughly absorbed by the layer. In practice, however, excess sample may be applied so that the absorbent layer becomes overly saturated with the sample. As a result, a layer of excess sample can collect on the bottom surface of the absorbent layer, causing the surface to become shiny. This shininess, called wet-through, can dramatically increase the reflectance of the absorbent layer, distorting the measurement of the reaction and providing misleading information to the patient.
A second variable that can affect the test results is the hematocrit of the patient""s blood, which is a measure of the relative amount of red blood cells and plasma in a blood sample. Because the hematocrit of a patient""s blood sample can vary, the absorption of the sample by the absorbent layer can vary as well, resulting in hematocrit-dependent measurements of the patient""s blood glucose concentration.
Thus, there is a need for controlling the absorption of a liquid sample by an absorbent layer and reducing the effect of hematocrit. The present invention satisfies these needs and provides related advantages as well.
The present invention provides a method for controlling the absorption of a liquid sample through an absorbent layer 2 by (a) providing an air gap 4c defined by the absorbent layer 2, at least one side wall 4d and a translucent window 6; and (b) applying the liquid sample to the absorbent layer on the side opposite to the air gap 2a. As a result, the sample absorption is controlled by preventing the release of air from the air gap 4c. 
The present invention also provides an apparatus for performing the method of the invention. The apparatus has an absorbent layer 2, at least one side wall 4d and a translucent window 6, where the layer, walls and window define an air gap 4c. 